A lithium-ion secondary battery comprises a material (an active material) which reversibly stores and releases lithium ions (Li ions) at positive and negative electrodes, and charging and discharging are realized by the migration of lithium ions between the positive and negative electrodes. Typical examples of an active material used in an electrode (typically, a positive electrode) of such a lithium-ion secondary battery include a complex oxide containing lithium and a transition metal element. For example, a lithium complex oxide (a nickel-containing lithium complex oxide) which at least contains nickel (Ni) as the transition metal element and which has a layered structure is preferably used. Patent Literature 1 is an example of a technical document relating to active materials of lithium-ion secondary batteries.
Patent Literature 1 discloses a positive electrode active material made of lithium-containing complex oxide powder whose DBP liquid adsorption ranges from 20 to 40 ml per 100 g of lithium-containing complex oxide powder weight. In this case, for the DBP liquid adsorption of the powder, a reagent solution of the powder is titrated with a constant-rate burette using an absorptometer, a plastograph, or a plasti-corder in compliance with test method A or test method B for DBP adsorption as defined in JIS K6217 (1997), a change in viscosity characteristics during the titration is measured by a torque sensor and recorded, whereby the DBP liquid adsorption of the powder is defined as an additive amount of the reagent solution corresponding to a torque that is 70% of a maximum generated torque.